Permanent magnet cylinder elements (e.g. cylinders and sections of cylinders) having radial polarization patterns, are used extensively in electrostatographic imaging apparatus. In one use, such cylinders function as magnetic brushes that transport magnetic developer (e.g. magnetic carrier particles with electrostatically attracted toner particles) into contact with the electrostatic images on a photoconductor. In another use such magnetic cylinders elements are used in scavenger rollers to remove any carrier particles that were erroneously deposited on the photoconductor along with the desired toner transfer.
In both the developer roller and scavenger roller systems, device constructions vary greatly. Magnetic brush devices can be constructed with a rotating magnetic cylindrical core within a stationary non-magnetic cylindrical shell or with a stationary magnetic core within a rotating non-magnetic shell, or both the magnetic core and the outer shell can be constructed to rotate, see e.g. U.S. Pat. No. 4,473,029. Scavenger roller devices also vary considerably in design approach.
Beyond the general design approach, the magnetic brush and scavenger devices will have design details that depend significantly on the characteristics of the overall electrostatographic system, e.g. its particular developer composition, the photoconductor speed, the strength of the electrostatic image pattern, etc. One of the very important design details of such devices is the magnetization pattern of their permanent magnet cylinder element(s), so that each different electrostatographic apparatus design benefits from a "custom designed" magnetization pattern for its developing and scavenging devices.
In one prior art approach, elongated strip magnets are attached at predetermined positions around the periphery of a cylindrical iron core to construct desired magnetization patterns. In another, a plurality of magnet elements are adhered together to form a cylinder with the desired magnetization pattern. In a more recent design approach, a cylinder formed of permanent magnet material such as molded ferrite is placed in a custom magnetizing fixture and impressed with the desired polarization pattern.
The last-noted approach is desirable from the viewpoint of material and assembly costs, once the appropriate fixture has been developed. However, the design and fabrication of the fixture is not an insignificant endeavor. For example, such prior art fixtures can comprise a block of phenolic or other suitable dielectric material, centrally bored to allow the magnet element to be magnetized to fit with a small clearance. A heavy gauge wire conductor is threaded through groups of holes drilled longitudinally through the block, adjacent to the central bore, in predetermined locations based upon the desired magnetization pattern to be achieved. The element to be magnetized is inserted in the bore and the wire ends are coupled to a capacitor discharge magnetizer. The entire assembly is then inserted into a water cooled shell and a high current pulse is directed through the wires to produce magnetic fields that magnetize the inserted element in the proper polarization pattern.
The magnetizing fixture described above is expensive to construct. Moreover, the interactions of the various instrumentalities make calculating the predesign of a fixture that will provide a precise polarization pattern, virtually impossible. Thus repeated fixtures designs are calculated and constructed in attempts to develop approximately the desired field by trial and error. At some stage a compromise is attained between the preciseness of the polarization patterns formed by a given fixture design and the cost of continuing to refine the design by constructing new fixtures. Whenever the overall machine design changes to necessitate a different preferred polarization pattern, it is necessary to repeat the process of fixture design as described above.
Considering the foregoing, one can appreciate that it would be highly desirable to improve the apparatus, systems and processes for impressing precise polarization patterns upon cylindrical permanent magnet elements.